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Old 7th June 2010, 11:23 AM   #1
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Default External power supply / dangerous voltage?

Hey guys,

This may be stupid questions, but I am currently building my ta2020 amp kit (made by Arjen Helder -25Watt stereo amp) and was thinking about buying a fitting power supply. As I wanted the housing as small as possible I decided to use an external power supply like that: PSC30R-120, Phihong PSC30R-120 Interchangeable Wall Plug

My questions are:

What do you think about external power supplies like that? Are they good enough or is it better to use an SMPS? Do you have any recommendations?

Is it possible to get an electrical shock with 12V and 2 Ampere? I mean 12V is nothing, but as 50mA could be lethal I was thinking about that too. Normally I would say that one would need way more voltage (up to 50 or more to compensate for the resistance of the human body), but as my last physics lesson is ages ago I simply wanted to ask that too, even if that means that a few guys are laughing out there

Greetings,

Haensel
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Old 7th June 2010, 04:24 PM   #2
DJPhil is offline DJPhil  United States
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Greetings,
I'm a bit new around here, but I'll take a shot at your questions. If I mess up too badly I trust someone will let us know.

Quote:
Originally Posted by Haensel View Post
What do you think about external power supplies like that? Are they good enough or is it better to use an SMPS? Do you have any recommendations?
Technically, this wall plugin is also a SMPS. SMPS stands for switched-mode power supply, and in the context of audio the other common option is a linear power supply. SMPS units come in all sizes, from on-chip converters for serial port powering to giant computer power supplies and above. The benefits and drawbacks to each are debatable, but in general SPMS units tend to be more compact, usually more efficient, and much lighter due to the absence of a low frequency transformer.

In your case this power supply might a close call. I don't have as much experience with SMPS as I do with linear supplies, so I don't know what sort of margin you should be looking for. Anyone with more experience want to weigh in?

Regarding noise, a quick peek at the TA2020 datasheet shows a PSRR (Power Supply Rejection Ratio) at a guaranteed 60dB minimum, and 80dB typical at 100mV. This is good for a chip amp, and means that it would take an exceptionally bad power supply to inject ripple into the output.

Looking at the power supply datasheet shows a max peak to peak ripple of 120mV, "Measured with by-pass capacitors 0.1uf//10uf at output connector terminal." Your schematic probably has at least this much bypass on the Vdd rail, as the datasheet reference circuit calls for 0.1uF and 180uF (Csw). It's probably unnecessary to increase the size of the bulk capacitor, but you can experiment to see if it helps with ripple rejection, but I'd try to keep it under 1000uF to avoid high inrush currents (opinion and guesswork). The smaller capacitor is for high frequency RF rejection, and should probably be left alone.


Quote:
Originally Posted by Haensel View Post
Is it possible to get an electrical shock with 12V and 2 Ampere? I mean 12V is nothing, but as 50mA could be lethal I was thinking about that too. Normally I would say that one would need way more voltage (up to 50 or more to compensate for the resistance of the human body), but as my last physics lesson is ages ago I simply wanted to ask that too, even if that means that a few guys are laughing out there
I'm a stickler about electrical safety, mainly because I'm clumsy as hell. There's a good writeup at allaboutcircuits that covers pretty much everything, as well as an excellent guide at the ESP audio pages, which is full of useful info on all sorts of things.

In essence, you are largely avoiding the majority of hazards by working with low voltages and not building your own power supply. One bit of caution that may not gel out of the safety information above: beware sharp things. If a conductor pierces your skin it's bypassed almost all of your protective resistance, and you'll essentially become a 500Ω load.

I hope that helps some, I do tend to ramble on.
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Old 8th June 2010, 11:25 AM   #3
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Hey DJPhil,

Thank you for your reply. The links are awesome: Normally topics like that are described in an overcomplicated, academic way. This is just what I needed. Simple and down to the facts

Thanks and greetings from Vienna/Austria
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Old 8th June 2010, 12:13 PM   #4
AndrewT is offline AndrewT  Scotland
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regards Andrew T.
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Old 8th June 2010, 03:34 PM   #5
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Since the powersupply ALWAYS is a part of the signal way through ANY analog audio circuit, there are certain aspects that you need to consider - first and foremost the capacitors at the output of the powersupply. They need to be big enough, since they have an absolute influence on the circuits bass responce !!!!
This goes for an SMPS supply too, although not all SMPS designs works equally well with big capacitors in the output (= Power Input to the amp).
The routing of the power and ground is extremely important too - in order to get a noise free amp.

Under normal circumstances you're noway near drawing 50mA through your body at 12v, for that you need a higher voltage. Since this is very individual (because of sweat, etc.) some authorities has selected 24v as the dividing line. Below that even the most sensitive individuals has no problems under normal circumstances and some requires several hundreds of volts if they just touch with their finger - DO NOT TRY THIS. I've witness this, but since it's potentially VERY DANGEROUS, don't try this.

Again under 24 volts you're normally in the clear as to electric-shock and some agecies sets the limit at 48 volts, but remember to respect any circuit that's capable of delivering big amounts of energy.
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Old 9th June 2010, 12:15 AM   #6
wwenze is offline wwenze  Singapore
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Let me try touching my computer's 12V (the PSU is capable of 29A), no, nothing.

Last edited by wwenze; 9th June 2010 at 12:17 AM.
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Old 9th June 2010, 01:19 AM   #7
jcx is offline jcx  United States
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google: selv safety
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